(1) Field of the Invention
The present invention relates to an optical method and device for detecting surface and structural defects of a travelling hot product, it is particularly applied to metal products during hot-rolling and to flat metal products at the exit of a hot-rolling mill.
(2) Prior Art
The metal products are generally obtained from slabs and billets processed in continuous casting and then hot-rolled in a succession of rolling stands. Rolling is performed at high temperature, generally towards 1,000° C. when dealing with steel. But the invention may also apply to non-ferrous metals such as copper or aluminium, as well as to plastics.
By practicing the rolling of steel, the section of the product may be reduced while lengthening it along the longitudinal axis which is the axis of travel during rolling. With this method, particular mechanical properties with a certain axial isotropy may be given to the product.
But the method is not without drawbacks for the structure of the product, particularly of metal products and especially steel. Indeed, defects of different origins may be found associated in the structure of the product, it is possible to have surface defects caused by scale which remains adhered to the surface of the product and which originates from the oxidization of the surface caused by the water cooling of the rolling stands. Encrusted scale may also be found inside the product. Other defects may stem from difficulties encountered further upstream, such as bad lubrication upon casting the slab or heat shock during the cooling. These hazards of the manufacturing method may generate structural defects in the metal, which may be localized inside the thickness of the product as well at its surface.
Being able to detect surface defects and defects present under the surface is therefore important for all the products. When optical devices for detecting defects are set up in the hot-rolling facility, the latter may be detected by observing the light emitted by the product itself or that stemming from an auxiliary illumination source and reflected by the product. For example, specially developed optical devices are already known for detecting surface defects of flat products, using means for illuminating the surface and means for observing the reflected light. Observation of the emitted light in the visible light region gives good results for detecting surface defects but does not allow detection of defects localized under the surface. The idea was then devised of associating the observation of the light emitted by the product, which emits radiation in the infrared region because of its temperature, and of that which is emitted by an additional illumination, generally in the visible region and reflected by the surface of the product. The surface defects are the only ones which are detected by reflection of additional illumination, and defects present under the surface may be detected by proceeding with subtraction, as they are transmitted by the infrared light emitted by the core of the product.
A well-known optical problem is then posed, which is that of chromatic aberration of the lenses. Indeed, the images taken with the different lights must be able to be processed and compared, the defects under the surface being derived from the processing of the difference between both images. For this, sharp and superposable images must be obtained.
The problem of chromatic aberration is solved in different ways in conventional devices for shooting images. In a device of this type, the phenomenon leads to having sharp images in different focal planes according to the wavelength. In cameras for example, the combination of convergent lenses and divergent lenses, for which the phenomenon is inverted, may allow an image focal plane to be built in which both effects are cancelled out. One then has a lens called an achromatic lens. The diaphragm may also be used in order to increase the depth of field. Indeed, the objective of an image shooting device is calculated for its full aperture, i.e., for processing the light rays arriving over its entire surface. If the diaphragm is closed, the diameter of the aperture letting through the light rays is limited and the objective only operates in its central area for which these phenomena are reduced and the field of sharpness increases.
This property is further called depth of field, this means that the range of distances for which the image will be sharp in the image focal plane, is extended. But, for this, sufficient light should be available and this kind of devices was proposed by the applicant company for detecting defects of long iron and steel products which are narrow and therefore generate a concentrated image containing sufficient light. This is different for flat products for which the width of the dimension to be observed is larger and the light is less concentrated.
It would also be possible to vary the other image shooting parameter: the exposure time. Indeed, these automatic inspection devices use cameras fitted with photosensitive sensors which operate with a certain integration time. With this trick, a sufficient signal is obtained in the case of faint light. But the integration time is limited by the travelling speed of the product at which the observation must be made while retaining a sharp image. Now, it is found that today, flat iron and steel products have final rolling speeds much larger than those of long products and they may attain 25 meters per second. Therefore a solution is therefore not in the integration time, because the intention today is to utilize automatic inspection devices operating on-line and at the production rate, this for minimizing costs.
In order to solve this problem of image formation in a different focal plane depending on the wavelength, certain camera manufacturers provide operation in the infrared by moving the focusing ring; often a red colour engraving is present on the focusing ring of the objectives. An empirical formula for calculating this displacement gives a shift of 0.0025 times the focal distance. But in fact this depends on the wavelength range used. The formula well-known to one skilled in the art, allows passing from visible light to a certain range of infrared light. But there exists a very extended range of this. Actually, the visible range is from 0.4 micrometer to 0.7 micrometer, there exists a near-infrared range between 0.7 and 3 micrometers, a medium infrared range between 3 and 25 micrometers and a far infrared range between 25 and 100 micrometers. All depends on the radiation from the body to be observed, therefore on its temperature. As regards the flat steel products at the exit of the hot-rolling mill, the temperature depends on the nature of the manufactured steel. The continual development of new touches of steel requires very accurate heat control during the hot-rolling and at the exit of the rolling mill up to the coiling of the strips, in particular for steels, the plasticity of which depends on their deformation, and which include phases in the metastable state. The outlet temperature may then currently vary today between 900° C. and 600° C. or less. It is therefore worthwhile to design a device which may also operate with far infrared light.
Furthermore, the methods proposed by camera manufacturers allow passing from visible light to a range of infrared light but the focusing for two images simultaneously derived from two lights cannot be obtained. Another solution would then be to use two cameras but the solution would be much more expensive, especially as colour cameras with three arrays for the three red, green and blue components of the usual cinema and television images are currently available, and as it is convenient to use an array in the infrared by simply setting up the adequate filter. Further, the method requires superposition of the images for processing contrasts by difference and this step of the method would be made much harder to achieve from images obtained from different cameras and objectives.